The present invention relates to an array of flow directing elements for turbomachinery, in particular to an improved rotor blade array having improved flutter resistance due to structural mistuning.
Turbomachinery devices, such as gas turbine engines and steam turbines, operate by exchanging energy with a working fluid using alternating rows of rotating blades and non-rotating vanes. Each blade and vane has an airfoil portion that interacts with the working fluid.
Airfoils have natural vibration modes of increasing frequency and complexity of the mode shape. The simplest and lowest frequency modes are typically referred to as first bending, second bending, and first torsion. First bending is a motion normal to the flat surface of an airfoil in which the entire span of the airfoil moves in the same direction. Second bending is similar to first bending, but with a change in the sense of the motion somewhere along the span of the airfoil, so that the upper and lower portions of the airfoil move in opposite directions. First torsion is a twisting motion around an elastic axis, which is parallel to the span of the airfoil, in which the entire span of the airfoil, on each side of the elastic axis, moves in the same direction.
It is known that turbomachinery blades are subject to destructive vibrations due to unsteady interaction of the blades with the working fluid. One type of vibration is known as flutter, which is an aero-elastic instability resulting from the interaction of the flow over the blades and the blades"" natural vibration tendencies. The lowest frequency vibration modes, first bending and first torsion, are typically the vibration modes that are susceptible to flutter. When flutter occurs, the unsteady aerodynamic forces on the blade, due to its vibration, add energy to the vibration, causing the vibration amplitude to increase. The vibration amplitude can become large enough to cause structural failure of the blade. The operable range, in terms of pressure rise and flow rate, of turbomachinery is restricted by various flutter phenomena.
It is also known that the blades"" susceptibility to flutter is increased if all blades on a disk are identical in terms of their vibration frequencies. Advances in manufacturing techniques have resulted in the production of blades that have nearly uniform properties. This uniformity is desirable to ensure consistent aerodynamic performance, but undesirable in that it increases susceptibility to flutter. Therefore, it has become desirable to introduce intentional variation in the blades during the manufacturing process to achieve flutter resistance. These variations should significantly affect the vibration characteristics of the blade, thus introducing structural mistuning, without compromising aerodynamic performance or introducing undue complexity to the manufacturing process.
The use of nonuniformity in vibration frequency to avoid flutter instability for a row of attached blades is addressed in U.S. Pat. No. 5,286,168 to Smith. The approach discussed in this patent uses frequency nonuniformity for flutter avoidance, but requires the manufacture of two distinct blade types.
The use of nonuniformity in shroud angle to avoid flutter instability for a blade row of attached, shrouded blades is addressed in U.S. Pat. No. 5,667,361 to Yaeger et al. This approach is unattractive for modern gas turbine engines since the use of shrouds imposes an aerodynamic performance penalty.
Accordingly, it is an object of the present invention to provide an improved array of flow directing elements for use in turbomachinery, which array provides passive flutter control.
It is a further object of the present invention to provide an improved array as above which does not require two distinct types of flow directing elements.
The foregoing objects are attained by the improved array of the present invention.
In accordance with the present invention, an array of flow directing elements for use in turbomachinery for providing passive flutter control is provided. The array broadly comprises a plurality of flow directing elements mounted to a rotor disk with said plurality of flow directing elements comprising a first set of first flow directing elements whose natural vibration frequency has been modified by having material removed from a leading edge tip region and a second set of second flow directing elements whose natural vibration frequency has been modified by having material removed from a midspan leading edge region.
Other details of the structurally mistuned array of the present invention, as well as other objects and advantages attendant thereto, are set forth in the following detailed description and the accompanying drawings wherein like reference numerals depict like elements.